I was always wondering why DJ served Beckett but didn`t feel that other two heartbeareres, Jack and Norrington, were in possession of the heart and, therefore, didn`t pledge his allegiance to any of them. I suspected that it may be the case of will to rule the world by controlling the heart. Deleted scene Map Is Finished confirms it`s the case. Don`t you hate when they omit important stuff?
Anyway, in MIF scene, Beckett says something to the effect of "why kill DJ when we can add another ship to the fleet" and at that moment, FD appears and DJ salutes to Beckett. So I`m thinking that threat to stab the heart (which is what those marines with bayonettes pointed at the heart were supposed to convey) isn`t enough to have DJ as a personal lapdog. Especially since stabbing the heart means robbing yourself off the power to control the seas(before they found out about CotFD catch which, again, wasn`t too appealing). It`s mutually exclusive. But it is the will to rule the world by wielding the power of the heart that does the trick. We saw Jack with a shard pointed at DJ`s heart and yet DJ laughed it off and stabbed Will without even trying to negotiate terms first. He also didn`t sense that Jack and Norrington had the heart for awhile, neither of whom wanted the power of the heart as a means of controlling the seas. Jack wanted to use it as a leverage, bail himself out of a bad deal and give it back. Norrington wanted to use it as a leverige to get pardon and privateer comission (which is a huge downgrade from being Naval Captain or Commodore) and he thought Beckett would order its destruction. But it was Beckett who actually summoned DJ by announcing that he is LOTH so to speak. Neither of previous heartbearers wanted to be LOTH because they didn`t want to use the power of the heart (means they weren`t corrupt). So, techincally, if M&M wanted to control the seas and ran away with the heart for the purpose of wieling its power as rulers of the world, they`d be able to summon DJ into their service.
I don`t think that Will`s heart has the power because Will isn`t corrupted. DJ`s power comes from corrupting his purpose. His role was only to ferry souls of the dead, not to control the seas cause it was Calypso`s purpose. He wasn`t the God of the Seas. IMO, his moping about romantic betrayal that resulted in binding Calypso to human form (so she cannot control the seas), is smoke and mirrors to a degree. Yes, he was hurt but he also wanted the divine power over seas that she had. So he took it from her and his heart became the object of power like the One Ring. OTOH, Will isn`t corrupted, doesn`t desire power to rule so if someone tried to use his heart, it wouldn`t work because there`s no evil magic. And he certainly wouldn`t do someone`s evil bidding. In short, one can be only Lord of the Corrupt Heart cause true one cannot have a master nor true hearts desire the power over everyone and everything.